During manufacture of integrated circuits, a wide variety of operating characteristics and circuit functions are tested. Because integrated circuit fabrication involves a number of process steps, variations in circuit parameters are commonplace. Thus, integrated circuits are commonly designed to include adjustable circuit parameters, in which adjustment of these parameters occurs following completed fabrication. For example, many integrated circuits include a voltage regulator which receives an externally applied supply voltage and produces a regulated supply voltage for use by other circuitry internal to the integrated circuit. The magnitude of this internal supply voltage is typically adjusted following completed fabrication of the integrated circuit to provide a regulated voltage at the appropriate operating level.
FIG. 1 depicts a voltage regulator 10 of the type used in a wide variety of integrated circuits. The voltage regulator 10 includes a voltage reference circuit 12 and a power stage circuit 14, whose configuration and operation are well known to those skilled in the art. The voltage reference circuit 12 receives an input voltage V.sub.IN which is a function of the externally applied supply voltage V.sub.CCX, as described below. The voltage reference circuit 12 then produces a reference voltage output V.sub.REF which is input to the power stage circuit 14. The power stage circuit 14 correspondingly produces a regulated supply voltage V.sub.CCR for powering other circuits internal to the integrated circuit in which the voltage regulator 10 is included.
Diodes D1 and D2 are connected in series between the external supply voltage V.sub.CCX and a node between the power stage circuit 14 and the voltage reference circuit 12. As will be understood by those skilled in the art, these diodes are used during burn-in testing, essentially clamping the voltage of the node to a fixed level below V.sub.CCX, once V.sub.CCX exceeds an expected operating range.
A plurality of transistors T1-T4 is connected in series with a resistor R between the external supply voltage V.sub.CCX and ground potential. As depicted, each of the transistors T1-T4 is a PMOS transistor with its gate connected to ground potential. Typically, these transistors have the same channel width and different channel lengths, and these transistors function essentially like resistors. Each of the transistors T1-T4 is connected to a corresponding one of shunting elements S1-S4, which are switching PMOS transistors connected in parallel with the corresponding transistor. In response to a corresponding one of signals FUSE1*-FUSE4* applied to its gate, each of the shunting elements S1-S4 can selectively electrically bypass the corresponding one of the transistors T1-T4, thereby selectively varying the resistance provided by the transistors. The input voltage V.sub.IN applied to the voltage reference circuit 12 is produced at a node between the resistor R and the transistors T1-T4. Depending on which of the transistors T1-T4 is electrically shunted, if any, the magnitude of the input voltage V.sub.IN is correspondingly adjusted. This affects the magnitude of the produced reference voltage V.sub.REF, which in turn affects the regulated internal supply voltage V.sub.CCR.
A trimming circuit 16 includes a plurality of programmable fuse elements F1-F4. Each of the fuse elements F1-F4 is connected in series with a corresponding one of a plurality of transistors 18 between the external supply voltage V.sub.CCX and ground potential. As depicted, each of the transistors 18 is a PMOS transistor with its gate tied to ground potential and acts as a pull-up transistor. The trimming circuit 16 also includes a plurality of inverters 20, each of which has its input connected to a node between the corresponding fuse element F1-F4 and transistor 18. The output signal produced by each of the inverters 20 is the corresponding one of the signals FUSE1*-FUSE4*. When, for example, the fuse element F1 is programmed (i.e., is blown), the input to the corresponding inverter 20 is held at a high logic state, and the signal FUSE1* is then asserted at a low logic state. When the fuse element F1 is not programmed, the input to the inverter 20 is then held at a logic low state, and the output signal FUSE1* is correspondingly deasserted.
During mass production of integrated circuits having a trimmable voltage regulator like that depicted in FIG. 1, measurements of the various producible V.sub.CCR magnitudes are recorded. Typically, fuse elements are blown and the resulting effect on regulator output is recorded in what is called a trim table. The trim table is usually created by hand measurement of the change in V.sub.CCR resulting from each fuse that is blown. For some relatively unstable fabrication processes, the resulting trim table can vary widely from lot to lot, wafer to wafer, and even from die to die. Creation of trim tables is time consuming, and many die may be tested to create an accurate trim table for an entire lot. Also, during creation of trim tables, the value of the regulated supply voltage V.sub.CCR is necessarily permanently altered on those die being tested. Thus, significant inefficiencies exist in current methods of testing and trimming integrated circuits.